重要な部分はSOMEFILENAMEの置き換えです
function MYMODULE_background_form_validate($form, &$form_state) {
$file = file_save_upload('file', array(
'file_validate_is_image' => array(),
'file_validate_extensions' => array('jpg'),
));
if ($file) {
if ($file = file_move($file, 'public://**SOMEFILENAME**', FILE_EXISTS_REPLACE)) {
$form_state['values']['file'] = $file;
}
else {
form_set_error('file', t('Failed to write the uploaded file the site\'s file folder.'));
}
}
else {
form_set_error('file', t('No file was uploaded.'));
}
}
function MYMODULE_background_form_submit(&$form, &$form_state) {
$file=$form_state['values']['file'];
unset($form_state['values']['file']);
$file->status = FILE_STATUS_PERMANENT;
file_save($file);
drupal_set_message(t('Thanks, the background has been saved.'));
}